Method and system for counting events within a simulation model

ABSTRACT

A method and system that utilize the expressiveness of hardware description languages for efficiently and comprehensively monitoring performance characteristics of a digital circuit design during simulation. According to the present invention, a design entity that is part of a digital circuit design is first described utilizing a hardware description language. Next, a counting instrument is described utilizing the same hardware description language. The counting instrument is designed to detect occurrences of a count event within the design entity during simulation of the digital circuit design. The counting instrument is associated with the design entity utilizing a non-conventional call, such that the counting instrument may be utilized to monitor each instantiation of the design entity within the simulation model without the instrumentation entity becoming incorporated into the digital circuit design. In association with the counting instrument, a linear feedback shift register is automatically generated for recording the number of occurrences of the count event within the design entity.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is related to the following copending U.S. Patent Applications: U.S. patent application Ser. No. 09/190,865 filed on Nov. 9, 1998, titled “Method And System For Instrumenting Simulation Models”; U.S. patent application Ser. No. 09/190,861 filed on Nov. 9, 1998, titled “Method And System For Incrementally Compiling Instrumentation Into A Simulation Model”; U.S. patent application Ser. No. 09/190,862 filed on Nov. 9, 1998, titled “Automatic Adjustment For Counting Instrumentation”; U.S. patent application Ser. No. 09/190,863 filed on Nov. 9, 1998, titled “Hardware Simulator Instrumentation”; and U.S. patent application Ser. No. 09/190,864 filed on Nov. 9, 1998, titled “Method And System For Selectively Disabling Simulation Model Instrumentation”. The above mentioned patent applications are assigned to the assignee of the present invention. The content of the cross referenced copending applications are hereby incorporated herein by reference thereto.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates in general to a method and system for interactively designing and simulating complex circuits and systems, particularly digital devices, 1modules and systems. In particular, the present invention relates to a method and system that utilize instrumentation modules written in hardware description language to monitor the operation of computer-generated digital circuit designs. More particularly, the present invention relates to an improved counting method by which events detected by HDL instrumentation entities may be counted without the need for excess incrementation logic.

2. Description of the Related Art

Verifying the logical correctness of a digital design and debugging the design, if necessary, are very important steps in most digital design processes. Logic networks are tested either by actually building networks or by simulating networks on a computer. As logic networks become highly complex, it becomes a necessary to simulate a design before the design is actually built. This is especially true when the design is implemented as an integrated circuit, since the fabrication of integrated circuits requires considerable time and correction of mistakes is quite costly. The goal of digital design simulation is the verification of the logical correctness of the design.

In a typical automated design process that is supported by a conventional electronic computer-aided design (ECAD) system, a designer enters a high-level description utilizing a hardware description language (HDL), such as VHDL, producing a representation of the various circuit blocks and their interconnections. The ECAD system compiles the design description into a format that is best suited for simulation. A simulator is then utilized to verify the logical correctness of the design prior to developing a circuit layout.

A simulator is typically a software tool that operates on a digital representation, or simulation model of a circuit, and a list of input stimuli representing inputs of the digital system. A simulator generates a numerical representation of the response of the circuit which may then either be viewed on the display screen as a list of values or further interpreted, often by a separate software program, and presented on the display screen in graphical form. The simulator may be run either on a general purpose computer or on another piece of electronic apparatus specially designed for simulation. Simulators that run entirely in software on a general purpose computer will hereinafter be referred to as “software simulators”. Simulators that are run with the assistance of specially designed electronic apparatus will hereinafter be referred to as “hardware simulators”.

Usually, software simulators perform a very large number of calculations and operate slowly from the user's point of view. In order to optimize performance, the format of the simulation model is designed for very efficient use by the simulator. Hardware simulators, by nature, require that the simulation model comprising the circuit description be communicated in a specially designed format. In either case, a translation from an HDL description to a simulation format, hereinafter referred to as a simulation executable model, is required.

Simulation has become a very costly and time-consuming segment of the overall design process as designs become increasingly complex. Therefore, great expense is invested to ensure the highest possible accuracy and efficiency in the processes utilized to verify digital designs. A useful method of addressing design complexity is to simulate digital designs at several levels of abstraction. At the functional level, system operation is described in terms of a sequence of transactions between registers, adders, memories and other functional units. Simulation at the functional level is utilized to verify the high-level design of high-level systems.

At the logical level, a digital system is described in terms of logic elements such as logic gates and flip-flops. Simulation at the logic level is utilized to verify the correctness of the logic design. At the circuit level, each logic gate is described in terms of its circuit components such as transistors, impedences, capacitances, and other such devices. Simulation at the circuit level provides detailed information about voltage levels and switching speeds.

VHDL is a higher level language for describing the hardware design of complex devices. The overall circuit design is frequently divided into smaller parts, hereinafter referred to as design entities, that are individually designed, often by different design engineers, and then combined in a hierarchical manner to create an overall model. This hierarchical design technique is very useful in managing the enormous complexity of the overall design. Another advantage of this approach is that errors in a design entity are easier to detect when that entity is simulated in isolation.

A problem arises however when the overall model is simulated as a whole. Compound errors may occur which mask other individual errors. Further, the enormity of modern digital design complexity makes the errors in each design entity difficult to recognize. Therefore, although the hierarchical nature of VHDL eases the development and modeling phases of complex designs, problems with obtaining accurate and comprehensive simulation test results of the overall design remain unresolved.

Therefore, there is a need to accurately monitor characteristics of specific modules or submodules of a large scale design in order to more efficiently and accurately diagnose problems with and assess the correctness of the overall design.

A current method of verifying large scale design models is to integrate programs written in high level languages such as C or C++ into the overall HDL design flow. Often, one or more custom-developed programs written in a high-level programming language are incorporated into the verification strategy as follows. The high level-language program or programs, hereinafter referred to as a reference model, are written and process test vectors to produce expected results. The reference model supplies the “expected correct result” of any given simulation run. The test vector is then run on the simulation execution model by the simulator. The results of the simulation run are then compared to the results predicted by the reference model. Discrepancies are flagged as errors. Such a simulation check is know by those skilled in the art as an “end-to-end” check. This method of “end-to-end” checking has two problems. First, the problem of masking of internal logic failures remains as these errors may not propagate to the final results of the circuit checked in an end-to-end test. Second, an end-to-end check may fail to catch an intermediate failure that occurred during the simulation run but was masked or overwritten by a later action in the simulation run.

A current method of overcoming these problems involves writing verification programs at the simulation phase of the design process that are designed to monitor, during the course of a simulation run, correctness characteristics and intermediate results. These verification programs are typically written in high level programming languages such as C or C++ since design languages such as HDLs are specialized to suit the needs of digital designers. Furthermore, languages such as C and C++ typically have greater expressiveness than HDL languages thereby facilitating the creation of complex checking programs. A problem associated with this method, however, is that it adds further complexity to the simulation process by requiring an extra communication step between designers and simulation programmers. The efficiency and effectiveness of the simulation testing are therefore reduced. Another problem with utilizing verification programs written in languages such as C++ or C is that these programs are not amenable to execution on a hardware simulator. In general, hardware simulators must be stopped after each simulation time period and the verification programs allowed to access the executable simulation model internals to perform their checking and monitoring functions. Such stoppages usually have a dramatic negative impact on the performance of hardware simulators.

Based on the foregoing, it can be appreciated that a need exists for a method and system that utilize the inherent hierarchical and modular nature of HDLs to provide simulation instrumentation in the form of HDL entities for digital circuit design simulation models. Such a method and system would be useful by permitting accurate monitoring of performance characteristics of specific modules or components of an overall model in order to more efficiently and accurately identify failures and assess the logical correctness of the overall model.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a method and system for interactively designing and simulating complex circuits and systems, particularly digital devices, modules and systems.

It is another object of the invention to provide a method and system for utilize instrumentation modules written in a hardware description language to monitor the performance of computer-generated digital circuit designs.

It is still another object of the invention to provide an improved counting method by which events detected by HDL instrumentation entities may be counted without the need for excess incrementation logic.

The above and other objects are achieved as is now described. A method and system are disclosed that utilize the expressiveness of hardware description languages for efficiently and comprehensively monitoring performance characteristics of a digital circuit design during simulation. According to the present invention, a design entity that is part of a digital circuit design is first described utilizing a hardware description language. Next, a counting instrument is described utilizing the same hardware description language. The counting instrument is designed to detect occurrences of a count event within the design entity during simulation of the digital circuit design. The counting instrument is associated with the design entity utilizing a non-conventional call, such that the counting instrument may be utilized to monitor each instantiation of the design entity within the simulation model without the instrumentation entity becoming incorporated into the digital circuit design. In association with the counting instrument, a linear feedback shift register is automatically generated for recording the number of occurrences of the count event within the design entity.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objects, and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein:

FIG. 1 illustrates a pictorial representation of a data processing system in which a preferred embodiment of the present invention may be implemented;

FIG. 2 depicts a representative hardware environment of the data processing system illustrated in FIG. 1;

FIG. 3A illustrates a simplified block diagram of a digital design entity in which a preferred embodiment of the present invention may be implemented;

FIG. 3B depicts a diagrammatic representation of a simulation model in which a preferred embodiment of the present invention may be advantageously utilized;

FIG. 3C is a flow diagram illustrative of a model build process in which a preferred embodiment of the present invention may be implemented;

FIG. 3D depicts a block diagram representative of data structures on which a preferred embodiment of the present invention may be utilized;

FIG. 4A illustrates a simplified block diagram of an instrumentation entity utilized in a preferred embodiment of the present invention;

FIG. 4B depicts a simplified block diagram of a simulation model instrumented in accordance with the teachings of the present invention;

FIG. 4C illustrates exemplary sections of HDL syntax utilized in a preferred embodiment of the present invention;

FIG. 4D is a flow diagram depicting a model build process in accordance with the teachings of the present invention;

FIG. 4E illustrates a block diagram representative of memory data structures in accordance with the teachings of the present invention;

FIG. 5A depicts a logic diagram of a runtime disable mechanism in accordance with a preferred embodiment of the present invention;

FIG. 5B illustrates a simplified block diagram of functional units utilized to execute the method and system of the present invention on a hardware simulator in accordance with the teachings of the present invention;

FIG. 6A depicts a simplified gate level representation of an exemplary counting instrument with a runtime disable feature and automatic clocking adjustment in accordance with the teachings of the present invention;

FIG. 6B is a simplified timing diagram illustrating operation of automatic clocking adjustment of counting instrumentation in accordance with a preferred embodiment of the present invention; and

FIG. 7 depicts an alternative counting means that may be employed for counting events detected by instrumentation entities in accordance with the teachings of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

The present invention accomplishes the goal of accurately and comprehensively monitoring the operation of a digital circuit design by allowing for designer creation of instrumentation modules in the same hardware description language (HDL) as utilized for the design. HDLs, while suited to the needs of digital designers can also be effectively utilized for a number of checking functions. By allowing a digital designer to utilize the HDL to create checking and instrumentation modules, the need to learn a language such as C or C++ is eliminated. These instrumentation modules will be utilized to monitor specified design parameters while not becoming an integral part of the design itself. Furthermore, since these instrumentation modules are written in the same HDL as utilized in the actual design, such modules are platform and simulator independent. Unlike checking done with C or C++ programs, HDL instrumentation can be compiled and run directly without loss of performance on hardware simulators.

With reference now to the figures and in particular with reference to FIG. 1, there is depicted a pictorial representation of a data processing system 10 with which the present invention may advantageously be utilized. As illustrated, data processing system 10 comprises a workstation 12 to which one or more nodes 13 are connected. Workstation 12 preferably comprises a high performance multiprocessor computer, such as the RISC System/6000 or AS/400 computer systems available from International Business Machines Corporation (IBM). Workstation 12 preferably includes nonvolatile and volatile internal storage for storing software applications comprising an ECAD system, which can be utilized to develop and verify a digital circuit design in accordance with the method and system of the present invention. As depicted, nodes 13 are comprised of a display device 14, a keyboard 16, and a mouse 20. The ECAD software applications executed within workstation 12 preferably display a graphic user interface (GUI) within display screen 22 of display device 14 with which a digital circuit designer can interact using a keyboard 16 and mouse 20. Thus, by entering appropriate inputs utilizing keyboard 16 and mouse 20, the digital circuit designer is able to develop and verify a digital circuit design according to the method described further hereinbelow.

FIG. 2 depicts a representative hardware environment of data processing system 10 illustrated in FIG. 1. In FIG. 1 and FIG. 2, like parts are identified by like numbers. Data processing system 10 in FIG. 2 is thus a configuration that includes all functional components of a computer and its associated hardware. Data processing system 10 includes a Central Processing Unit (“CPU”) 24, such as a conventional microprocessor, and a number of other units interconnected via system bus 26. CPU 24 includes a portion of data a processing system 10 that controls the operation of the entire computer system, including executing the arithmetical and logical functions contained in a particular computer program. Although not depicted in FIG. 2, CPUs such as CPU 24 typically include a control unit that organizes data and program storage in a computer memory and transfers the data and other information between the various parts of the computer system. Such CPUs also generally include an arithmetic unit that executes the arithmetical and logical operations, such as addition, comparison, multiplications and so forth. Such components and units of data processing system 10 can be implemented in a system unit such as workstation 12 of FIG. 1.

Data processing system 10 further includes random-access memory (RAM) 28, read-only memory (ROM) 30, display adapter 32 for connecting system bus 26 to display device 14, and I/O adapter 34 for connecting peripheral devices (e.g., disk and tape drives 33) to system bus 26. RAM 28 is a type of memory designed such that the location of data stored in it is independent of the content. Also, any location in RAM 28 can be accessed directly without having to work through from the beginning. ROM 30 is a type of memory that retains information permanently and in which the stored information cannot be altered by a program or normal operation of a computer.

Display device 14 is the visual output of data processing system 10. Display device 14 can be a cathode-ray tube (CRT) based video display well-known in the art of computer hardware. However, with a portable or notebook-based computer, display device 14 can be replaced with a liquid crystal display (LCD) based or gas plasma-based flat-panel display. Data processing system 10 further includes user interface adapter 36 for connecting keyboard 16, mouse 20, speaker 38, microphone 40, and/or other user interface devices, such as a touch-screen device (not shown), to system bus 26. Speaker 38 is one type of audio device that may be utilized in association with the method and system provided herein to assist diagnosticians or computer users in analyzing data processing system 10 for system failures, errors, and discrepancies. Communications adapter 42 connects data processing system 10 to a computer network. Although data processing system 10 is shown to contain only a single CPU and a single system bus, it should be understood that the present invention applies equally to computer systems that have multiple CPUs and to computer systems that have multiple buses that each perform different functions in different ways.

Data processing system 10 also includes an interface that resides within a machine-readable media to direct the operation of data processing system 10. Any suitable machine-readable media may retain the interface, such as RAM 28, ROM 30, a magnetic disk, magnetic tape, or optical disk (the last three being located in disk and tape drives 33). Any suitable operating system and associated interface (e.g., Microsoft Windows) may direct CPU 24. For example, the AIX operating system and AIX Windows windowing system can direct CPU 24. The AIX operating system is IBM's implementation of the UNIX™ operating system. Other technologies also can be utilized in conjunction with CPU 24, such as touch-screen technology or human voice control.

Those skilled in the art will appreciate that the hardware depicted in FIG. 2 may vary for specific design and simulation applications. For example, other peripheral devices such as optical disk media, audio adapters, or chip programming devices, such as PAL or EPROM programming devices well-known in the art of computer hardware and the like, may be utilized in addition to or in place of the hardware already depicted. In addition, main memory 44 is connected to system bus 26, and includes a control program 46. Control program 46 resides within main memory 44, and contains instructions that, when executed on CPU 24, carries out the operations depicted in FIG. 4D and FIG. 4E described herein.

It is important to note that, while the present invention has been, and will continue to be, described in the context of a fully functional computer system, those skilled in the art will appreciate that the present invention is capable of being distributed as a program product in a variety of forms, and that the present invention applies equally regardless of the particular type of signal-bearing media utilized to actually carry out the distribution. Examples of signal-bearing media include: recordable-type media, such as floppy disks, hard disk drives, and CD ROMs, and transmission-type media such as digital and analog communication links.

Simulation models of digital circuit designs are comprised of at least one and usually many sub-units referred to hereinafter as design entities. FIG. 3A illustrates a block diagram representation of an exemplary design entity 300 on which the method and system of the present invention may be implemented. Design entity 300 is defined by a number of components: an entity name, entity ports, and a representation of the function performed by design entity 300. Each entity within a given model has a unique name, not explicitly shown in FIG. 3A, that is declared in the HDL description of each entity. Furthermore, each entity typically contains a number of signal interconnections, known as ports, to signals outside the entity. These outside signals may be primary input/outputs (I/Os) of an overall design or signals connecting to other entities within an overall design.

Typically, ports are of one of three distinct types: input ports, output ports, and bi-directional ports. Design entity 300 of FIG. 3A is depicted as having a number of input ports 303 that convey signals into design entity 300. Input ports 303 are connected to input signals 301. In addition, design entity 300 is depicted as having a number of output ports 306 that convey signals out of design entity 300. Output ports 306 are connected to a set of output signals 304. Bi-directional ports 305 are utilized to convey signals into and out of design entity 300. Bi-directional ports 305 are in turn connected to a set of bi-directional signals 309. An entity, such as design entity 300, need not contain ports of all three types, and in the degenerate case, contains no ports at all. To accomplish the connection of entity ports to external signals, a mapping technique, known as a “port map”, is utilized. A port map (not explicitly depicted in FIG. 3A), consists of a specified correspondence between entity port names and external signals to which the entity is connected. When building a simulation model, ECAD software is utilized to connect external signals to appropriate ports of the entity according to a port map specification.

Finally, design entity 300 contains a body section 308 that describes one or more functions performed by design entity 300. In the case of a digital design, body section 308 contains an interconnection of logic gates, storage elements, other devices, and instantiations of other entities. By instantiating another entity within an entity, a hierarchical description of an overall design is achieved. For example, a microprocessor may contain multiple instances of an identical functional unit. In such a circumstance, it will often be the case that the microprocessor itself will be modeled as a single entity. Within the microprocessor entity, multiple instantiations of any duplicated functional entities will be present.

Referring now to FIG. 3B, there is illustrated a diagrammatic representation of an exemplary simulation model 329 in which a preferred embodiment of the present invention may be advantageously utilized. Simulation model 329 consists of multiple hierarchical entities. For clarity, the ports and signals interconnecting the entities within simulation model 329 have not been shown explicitly. In any model, one and only one entity is the so called “top-level entity”. A top-level entity 320, is that entity which encompasses all other entities within simulation model 329. That is to say, top-level entity 320 instantiates, either directly or indirectly, all descendant entities within a design. Simulation model 329 consists of top-level entity 320 which directly instantiates two instances, 321 a and 321 b of an FXU entity 321 and a single instance of an FPU entity 322. Each instantiation has an assocatiated description which contains an entity name and a unique instantiation name. For top-level entity 320, a description 310 is illustrated as “TOP:TOP”. Description 310 includes an entity name 312, depicted as the “TOP” preceeding the colon, and also includes an instantiation name 314, depicted as the “TOP” following the colon.

It is possible for a particular entity to be instantiated multiple times such as is depicted with instantiations 321 a and 321 b of FXU entity 321.

Instantiations 321 a and 321 b are distinct instantiations of FXU entity 321 with instantiation names FXU0 and FXU1 respectively. Top level entity 320 is at the highest level within the hierarchy of simulation model 329. An entity that instantiates a descendant entity will be referred to hereinafter as an ancestor of the descendant entity. Top-level entity 320 is therefore the ancestor that directly instantiates FXU entity instantiations 321 a and 321 b. At any given level of a simulation model hierarchy, the instantiation names of all instantiations must be unique.

In addition to FXU entity instantiations 321 a and 321 b, top level entity 320 directly instantiates a single instance of a FPU entity 322, with entity name FPU and instantiation name FPUO. Within an entity description, it is common for the entity name to match the instantiation name when only one instance of that particular entity is placed at a given level of a simulation model hierarchy. However, this is not required as shown by entity 322 (instantiation name FPU0, entity name FPU).

Within instantiation 321 a of FXU entity 321, single instance entities 325 a and 326 a of entity A 325 and entity B 326 respectively, are directly instantiated. Similarly instantiation 321 b of the same FXU entity contains instantiations 325 b and 326 b of entity A 325 and entity B 326 respectively. In a similar manner instantiation 326 a and instantiation 326 b each directly instantiate a single instance of entity 327 a and 327 b respectively, of entity C 327. The nesting of entities within other entities can continue to an arbitrary level of complexity provided that all entities instantiated, whether singly or multiply, have unique entity names and the instantiation names at any given level of the hierarchy are unique with respect to one another. Each entity is constructed from one or more HDL files that contain the information necessary to describe the entity.

Associated with each entity instantiation is a so called “instantiation identifier”. The instantiation identifier for a given instantiation is a string consisting of the enclosing entity instantion names proceeding from the top level entity instantiation name. For example, the instantiation identifier of instantiation 327 a of entity C 327 within instantiation 321 a of FXU entity 321 is “TOP.FXUO.B.C”. This identifier serves to uniquely identify each instantiation with a simulation model.

Referring now to FIG. 3C, there is depicted a flow diagram of one possible model build process in which a preferred embodiment of the present invention may be implemented. The process begins with one or more design entity HDL source code files 340 and, potentially, one or more design entity intermediate format files 345, hereinafter referred to as “proto files” 345, available from a previous run of HDL compiler 342. HDL compiler 342 processes HDL file(s) 340 beginning with the top level entity of a simulation model and proceeding in a recursive fashion through all HDL or proto file(s) describing a complete simulation model. During the process of compilation, HDL compiler 342, for each of HDL files 340, examines proto files 345 to determine if a previously compiled proto file is available and consistent. If such a file is available and consistent, HDL compiler 342 will not recompile that particular file, but will rather refer to the already extant proto file. If no such proto file is available or said proto file is not consistent, HDL compiler 342 will recompile the HDL file 340 in question explicitly and create a proto file 344, for use in subsequent compilations. Such a process will be referred to hereinafter as “incremental compilation” and can greatly speed the process of creating a simulation executable model 348. Incremental compilation will be shown in greater detail hereinbelow. Proto files 344, once created by HDL compiler 342, are available to serve as proto files 345 in subsequent compilations.

In addition to proto files 344, HDL compiler 342 also creates two sets of data structures, design entity proto data structures 341 and design entity instance data structures 343, in memory 44 of computer system 10. Design entity proto data structures 341 and design entity instance data structures 343, described hereinbelow in greater detail, serve as a memory image of the contents of a simulation executable model 348. Data structures 341 and 343 are passed, via memory 44, to a model build tool 346 for processing. Model build tool 346 processes data structures 341 and 343 into simulation executable model 348.

Hereinafter, for clarity of exposition, it will be assumed that each entity is described by a single HDL file. Depending on convention or the particular HDL in which the current invention is practiced, this restriction may be required. However, in certain circumstances or for certain HDLs it is possible to describe an entity by utilizing more than one HDL file. Those skilled in the art will appreciate and understand the extensions necessary to practice the present invention if entities are permitted to be described by multiple HDL files. Furthermore, it will be assumed that there is a direct correspondence, for each entity, between the entity name and both of the following: the name of the HDL file representing the entity and the name of the proto file for the entity.

In the following description, an HDL source code file corresponding to a given entity will be referred to by an entity name followed by “.vhdl”. For example, the HDL source code file that describes top level entity 320, will be referred to as TOP.vhdl. This labeling convention serves as a notational convenience only and should not be construed as limiting the applicability of the present invention to HDLs other than VHDL.

Returning to FIG. 3B, it can be seen that each entity may instantiate, either directly or indirectly, one or more other entities. For example, the FXU entity directly instantiates A entity 325 and B entity 326. Furthermore, B entity 326 directly instantiates C entity 327. Therefore, FXU entity 321 instantiates, directly or indirectly, A entity 325, B entity 326 and C entity 327. For clarity, those entities, that are directly or indirectly instantiated by another entity, will be referred to hereinafter as “descendants”. Therefore, the descendants of top level entity 320 are FXU entity 321, FPU entity 322, A entity 325, B entity 326, and C entity 327. It can be seen that each entity has a unique set of descendants and each time an entity is instantiated, a unique instance of the entity and its descendants is created. For example in simulation model 329, FXU entity 321 is instantiated twice, FXU:FXU0 321 a and FXU:FXU1 321 b, by top level entity 320. Each instantiation of FXU entity 321 creates a unique set of instances of the FXU, A, B, and C entities.

For each entity, it is possible to define what is referred to as a “bill-of-materials” or BOM. A BOM is a list of HDL files, with date and time stamps, of the entity itself and the entity's descendants. Referring again to FIG. 3C, the BOM for an entity is stored in proto file 344 for the entity created when HDL compiler 342 compiles HDL file 340 for the entity. In other words, when HDL compiler 342 compiles a particular HDL source code file among HDL files 340, a proto file 344 is generated that contains, among other components, a BOM listing the HDL files 340 that constitute the entity and the entity's descendants, if any. The BOM also contains the date and time stamp for each of the HDL files referenced as each appeared on disk/tape 33 of computer system 10 when the HDL file was being compiled.

In this manner, if any of the HDL files constituting an entity or the entity's descendants is subsequently changed, proto file 344 will be flagged as inconsistent and HDL compiler 342 will recompile HDL file 340 on a subsequent re-compilation as will be shown below. For example, going back to FIG. 3B, the HDL files referenced by the BOM of FXU entity 321 are FXU.vhdl, A.vhdl, B.vhdl and C.vhdl, each with appropriate date and time stamps. The files referenced by the BOM of top level entity 320 are TOP.vhdl, FXU.vhdl, A.vhdl, B.vhdl, C.vhdl, and FPU.vhdl with appropriate date and time stamps.

Returning to FIG. 3C, HDL compiler 342 creates an image of the structure of a simulation model in main memory 44 of computer system 10. This memory image is comprised of two major types of components: “proto” data structures 341 and “instance” data structures 343. A proto is a data structure that, for each entity in the model, contains information about the ports of the entity, the body contents of the entity, and a list of references to other entities directly instantiated by the entity (in what follows, the term “proto” will be utilized to refer to the in-memory data structure described above and the term “proto file” will be utilized to describe intermediate format file(s) 344). Proto files 344, produced by HDL compiler 342, are on-disk representations of the in-memory proto data structure.

An instance data structure is a data structure that, for each instance of an entity within a model, contains the instance name for the instance, the name of the entity the instance refers to, and finally the port map information necessary to interconnect the entity with external signals. During compilation, each entity will have only one proto data structure, while, in the case of multiple instantiations of an entity, each entity may have one or more instance data structures.

In order to incrementally compile a model efficiently, HDL compiler 342 follows a recursive method of compilation in which successive entities of the model are considered and loaded from proto files 345 if such files are available and are consistent with the HDL source files constituting those entities and their descendants. For each entity that cannot be loaded from existing proto files 345, HDL compiler 342 recursively examines the descendant entities of the entity, loading those descendant entities available from proto file(s) 345 and creating, as needed, proto files 344 for those descendant entities that are inconsistent with proto files 345. Psuedocode for the main control loop of HDL compiler 342 is shown below (the line numbers to the right of the psuedocode are not a part of the psuedocode, but merely serve as a notational convenience).

process_HDL_file(file) 5 { 10 if (NOT proto_loaded(file)) { 15 if (exists_proto_file(file) AND check_bom(file)) { 20 load_proto(file); 25 } else { 30 parse_HDL_file(file) 35 for (all instances in file) { 40 process_HDL_file(instance); 45 } 50 create_proto(file); 55 write_proto_file(file); 60 } 65 } 70 create_instance(file): 75 } 80

When compiler 342 is initially invoked, no proto data structures 341 or instance data structures 343 are present in memory 44 of computer system 10. The main control loop, routine process_HDL_file( ) (line 5), is invoked and passed the name of the top level entity by means of parameter “file”. The algorithm first determines if a proto data structure for the current entity is present in memory 44 by means of routine proto_loaded( ) (line 15). Note that the proto data structure for the top level entity will never be present in memory because the process starts with no proto data structures loaded into memory 44. If a matching proto data structure is present in memory 44, instance data structures for the current entity and the current entity's descendants, if any, are created as necessary in memory 44 by routine create_instance( ) (line 75).

However, if a matching proto data structure is not present in memory 44, control passes to line 20 where routine exists_proto_file( ) examines proto files 345 to determine if a proto file exists for the entity. If and only if a matching proto file exists, routine check_bom( ) is called to determine if proto file 345 is consistent. In order to determine if the proto file is consistent, the BOM for the proto file is examined. Routine check_bom( ) examines each HDL source code file listed in the BOM to determine if the date or time stamps for the HDL source code file have changed or if the HDL source code file has been deleted. If either condition occurs for any file in the BOM, the proto file is inconsistent and routine check_bom( ) fails. However, if check_bom( ) is successful, control is passed to line 25 where routine load_proto( ) loads the proto file and any descendant proto files into memory 44 creating proto data structures 341 for the current entity and the current entity's descendants, if any. The construction of process_HDL_file( ) ensures that once a proto file has been deemed consistent, all of its descendant proto files, if any, are also consistent.

If, however, the proto file is either non-existent or is not consistent, control passes to line 35 where routine parse_HDL_file( ) loads the HDL source code file for the current entity. Routine parse_HDL_file( ) (line 35) examines the HDL source code file for syntactic correctness and determines which descendant entities, if any, are instantiated by the current entity. Lines 40, 45, and 50 constitute a loop in which the routine process_HDL_file( ) is recursively called to process the descendent entities that are called by the current entity. This process repeats recursively traversing all the descendants of the current entity in a depth first fashion creating proto data structures 341 and proto data files 344 of the descendants of the current entity. Once the descendant entities are processed, control passes to line 55 where a new proto data structure is created for the current entity in memory 44 by routine create_proto( ). Control then passes to line 60 where a new proto file 344, including an associated BOM, is written to disk 33 by routine write_proto_file( ). Finally, control passes to line 75 where routine create_instance( ) creates instance data structures 343 for the current entity and any descendant entities as necessary. In this manner, process_HDL_file( ) (line 5) recursively processes the entire simulation model creating an in-memory image of the model consisting of proto data structures 341 and instance data structures 343.

With reference now to FIG. 3D there is depicted a block diagram representative of compiled data structures on which a preferred embodiment of the present invention may be implemented. Memory 44 contains proto data structures 361, one for each of the entities referred to in simulation model 329. In addition, instantiations in simulation model 329 are represented by instance data structures 362. Instance data structures 362 are connected by means of pointers indicating the hierarchical nature of the instantiations of the entities within simulation model 329. Model build tool 346 processes the contents of memory 44 in memory data structures in order to produce simulation executable model 348.

In order to instrument simulation models, the present invention makes use of entities known as “instrumentation entities”. An instrumentation entity is an entity with specific characteristics, as described below. An instrumentation entity is written utilizing the same HDL utilized to describe the entities in the design. In contrast to instrumentation entities, the entities constituting a design will hereinafter be referred to as “design entities”. Instrumentation entities, as with design entities are described by one or more HDL source code files and consist of a number of signal ports, a body section, and an entity name. In what follows, it will be assumed that an instrumentation entity is described by a single HDL file. Those skilled in the art will appreciate and understand extensions necessary to practice the current invention for an instrumentation entity that is described by multiple HDL files.

Each instrumentation entity is associated with a specific design entity referred to hereinafter as the “target entity”. The mechanism of the current invention provides a means of creating and connecting an instance of an instrumentation entity to each instance of the target entity within a simulation model.

With reference now to FIG. 4A there is illustrated a representation of an instrumentation entity 409. As with a design entity, instrumentation entity 409 has a number of input ports 400 which are connected, as described below, to signals 401 within the target entity which is not depicted in FIG. 4A. A body section 402 of instrumentation entity 409, contains logic necessary to detect occurrences of certain “events” on signals 401 of the target entity. A preferred embodiment of the present invention provides for monitoring three distinct types of events: “count” events, “fail” events, and “harvest” events, each described below in turn. Body section 402 contains internal logic which instrumentation entity 409 utilizes to detect occurrences of these events. A set of multi-bit output ports 403, 404, and 405 are connected to external instrumentation logic not depicted in FIG. 4A by to means of external signals 406, 407, and 408. Output ports 403, 404, and 405 provide the connection from the internal logic in body section 402 to the external instrumentation logic which is utilized to indicate the occurrence of count, failure and harvest events.

The method and system of the present invention provides for the monitoring of three distinct types of events within a simulation model. The first of these events is a failure event. A failure event is a sequence of signal values that indicate a failure in the correct operation of the simulation model. Each instrumentation entity can monitor the target entity for any desired number of failure events. Each failure occurrence is assigned to a particular signal bit on output port 403. Input ports 400 are connected to body section 402 containing logic necessary to detect the occurrence of the desired failure events. The logic of body section 402 produces an active high pulse on the correct bit of signal 403 when a failure event is detected. This error indication is conveyed by means of external signal 406 to external instrumentation logic, depicted in FIG. 4B as external instrumentation logic block 420, which flags the occurrence of the failure event.

In contrast to a failure event, a count event is a sequence of signal values that indicate the occurrence of an event within a simulation model for which it would be advantageous to maintain a count of the number of occurrences. Count events are utilized to monitor the frequency of occurrence of specific sequences within a simulation model. Each instrumentation entity can monitor the target entity for any desired number of count events. Each count event is assigned to a particular signal bit on output port 405. Input ports 400 are connected to logic block 402 containing the logic necessary to detect the occurrence of the desired count events. The logic of block 402 produces an active high pulse on the correct bit of signal 405 when a count event is detected. This count indication is conveyed by means of external signal 408 to instrumentation logic which contains counters utilized to record the number of occurrences of each count event.

Finally, a harvest event is a sequence of signal values that indicate the occurrence of a specific circumstance which would be advantageous to be able to reproduce. When a harvest event occurs, a register is loaded to indicate at what point within a simulation run the event occurred and a flag is set to indicate the occurrence of the specific circumstance. This allows the details of the simulation run to be saved in order to be able to recreate the specific circumstance monitored by the harvest event. Input ports 400 are connected to logic block 402 containing the logic necessary to detect the harvest events.

Each instrumentation entity can detect any desired number of harvest events. Each harvest event is assigned to a particular signal bit on output port 404. The logic of block 402 produces an active high pulse on the correct bit of signal 404 when a harvest event is detected. This harvest indication is conveyed by means of external signal 407 to external instrumentation logic which contains a register and flag for each harvest event. The register is utilized to record at which point in the simulation run the event occurred and the flag is utilized to indicate the occurrence of the event. One skilled in the art will recognize that it is possible to define additional types of events beyond those described above.

Instrumentation entities need not monitor for all three types of events. An instrumentation entity may monitor any non-empty subset of the three event types. Any design entity in a simulation model may be a target entity. Also, it is possible for more than one instrumentation entity to be associated with a particular design entity.

With reference now to FIG. 4B, wherein is depicted a representation of simulation model 329 instrumented in accordance with the teachings of the present invention. As can be seen in FIG. 4B, an instance 410 and an instance 411 of instrumentation entity FXUCHK are depicted monitoring instances 321 a and 321 b of an FXU entity. For each instantiation of FXU entity, 321 a and 321 b, an instantiation, 410 and 411 respectively, of FXUCHK is automatically generated by the mechanism of the present invention. In a similar fashion, instrumentation entity FPUCHK 412, is instantiated to monitor FPU entity 322. It should be noted that a significant advantage of the present invention is the automatic instantiation of instrumentation entities for each instance of a target entity.

Each instrumentation entity can monitor any signal within its associated target entity. In the example of FIG. 4B, entity FXUCHK is depicted as monitoring a signals Q 372, a signal R 376, and a signal S 374 within each of instances 321 a and 321 b of the FXU entity. Signal Q 372, is a signal within the instances 325 a and 325 b of descendant entity A. Likewise, signal S 374 is a signal within descendant entity C which is within descendant entity B. Finally, signal R 376 occurs directly within FXU entity 321. An instrumentation entity may monitor any signal within a target entity or the target entity's descendent entities. However, signals outside the target entity cannot be monitored.

Each instrumentation entity is connected by means of fail, count, and harvest signals to an instrumentation logic block 420. Instrumentation logic block 420 is generated automatically by the mechanism of the present invention and contains logic for recording occurrences of each of the three event types. For the count events monitored in simulation model 329, a set of counters 421 are utilized to count the number of occurrences of each count event. In a similar manner, a set of flags 424 is utilized to record the occurrence of failure events. Finally, a set of counters 422 and flags 423 are combined and utilized to record the point at which a harvest event occurs and the fact that a harvest event occurred, respectively. In one possible embodiment of the present invention, a cycle number is captured and stored utilizing counters 422 and flags 423 to record a harvest event. The logic structures of instrumentation logic block 420 are created by the mechanism of the present invention without direct intervention by the user.

In order to allow the method and system of the present invention to instantiate and connect instances of instrumentation entities, instrumentation entity HDL source code files include a specialized comment section, hereinafter referred to as “instrumentation entity description” , in a particular form that indicate the target entity for the instrumentation entity, the signals within the target entity to be monitored, and information for the different types of events monitored. It will be appreciated by those skilled in the art that it would be possible to directly extend the syntax of an HDL to support the encoding of this information without resorting to the use of comments as is described here.

With reference now to FIG. 4C there is illustrated an exemplary HDL file 440 that describes instrumentation entity FXUCHK depicted in FIG. 4B. HDL file 440 utilizes the syntax of the VHDL hardware description language. In the VHDL language, lines beginning with two dashes, “--”, are recognized by a compiler as being comment lines. The method and system of the present invention utilize comments of a non-conventional form to indicate information about an instrumentation entity. It will be appreciated by those skilled in the art that the exemplary syntax utilized in FIG. 4C is but one of many possible formats. FIG. 4C depicts one embodiment of the present invention in which comment lines that are utilized by the method and system of the present invention, begin with two exclamation points in order to distinguish these comments from conventional comments in instrumentation HDL file 440.

Within HDL file 440, the I/O ports of a FXUCHK entity are declared in entity declaration 450. Within entity declaration 450, three input ports, S_IN, Q_IN, and R_IN, respectively, are declared. Input ports, S_IN, Q_IN, and R_IN, will be attached to signal S, 374, signal Q, 372, and signal R, 376 respectively as described below. Input port, CLOCK, is also declared and will be connected to a signal, CLOCK, within the FXU entity and is as described below. In addition, three output ports: fails (0 to 1), counts(0 to 2), and harvests(0 to 1), are declared. These output ports provide failure, count, and harvest signals for two failure events, three count events, and two harvest events. In a preferred embodiment of the present invention, the names of the output ports are fixed by convention in order to provide an efficient means for automatically connecting these signals to instrumentation logic block 420.

A set of instrumentation entity descriptors 451 are utilized to provide the mechanism of the present invention information about the instrumentation entity. As seen in FIG. 4C, descriptor comments 451 may be categorized in a number of distinct sections: prologue and entity name declaration 452, an input port map 453, a set of failure message declarations 454, a set of counter declarations 455, a set of harvest declarations 456, and an epilogue 457.

The prologue and entity name 452 serve to indicate the name of the particular target entity that the instrumentation entity will monitor. Prolog and entity name declaration 452 also serves as an indication that the instrumentation entity description has begun. Specifically, the comment “--!! Begin” within prologue and entity name 452, indicates that the description of an instrumentation entity has begun. The comment “--!! Design Entity: FXU” identifies the target entity of the instrumentation entity which in HDL file 440 of FIG. 4C is design entity FXU. In a preferred embodiment of the present invention, this declaration serves to bind the instrumentation entity to the target entity.

Input port map 453 serves as a connection between the input ports of an instrumentation entity and the signals to be monitored within the target entity. The comments begin with comment “--!! Inputs” and end with comment “--!! End Inputs”. Between these comments, comments of the form “--!! inst_ent_port_name=>trgt_ent_signal_name” are utilized, one for each input port of the instrumentation entity, to indicate connections between the instrumentation entity ports and the target entity signals. The inst_ent_port_name is the name of the instrumentation entity port to be connected to the target entity signal. The trgt_ent_signal_name is the name of the signal within the target entity that will be connected to the instrumentation entity port.

It may be the case that a signal to be monitored lies within a descendant of a target entity. This is the case for signal S 374 which is embedded within entity C which is a descendant of entity B 326 and target FXU entity 321. Input port map 453 an identification string for signal S 374 which includes the instance names of the entities within the target entity each separated by periods (“.”). This identification string is pre-pended to the signal name. The signal mapping comment within input port map 453 for signal S 374 is therefore as follows:

--!!S ₁₃ IN=>B.C.S;

This syntax allows an instrumentation entity to connect to any signal within the target entity or the target entity's descendant entities. Signals, such as R 376, appearing on the top level of the target design entity have no pre-pended entity names:

--!!R _(—) IN=>R;

One, and only one, signal mapping comment must be provided for each input port of the instrumentation entity.

Failure message declarations 454 begin with a comment of the form “--!! Fail Outputs;”, and end with a comment of the form “--!! End Fail Outputs;”. Each failure event output is associated with a failure message. This message may be output by the simulation run-time environment upon detecting a failure event. Each failure event signal may be declared by a comment of the form “--!! n: “failure message”;” where n is an integer denoting which failure event the message is to be associated with and “failure message” is the message to be associated with the particular failure event. One, and only one failure message declaration comment must be provided for each failure event monitored by the instrumentation entity.

Counter declaration comments 455 begin with a comment of the form “--!! Count Outputs;”, and end with a comment of the form “--!! End Count Outputs;”. Each count event output is associated with a unique variable name. This name is associated with a counter in counter logic 421. The variable name provides a means to identify and reference the particular counter associated with a particular count event. A comment of the form “--!! n : <varname> qualifying_signal [+/−];” is associated with each counter event output. Within this convention, n is an integer denoting which counter event in the instrumentation module is to be associated with a variable name “varname” and qualifying_signal is the name of a signal within a target design entity utilized to determine when to sample the count event pulse as will be further described hereinbelow. The parameter “qualifying_signal” is followed by “+/−” to determine whether the qualifying signal will be a high active qualifying signal or a low active qualifying signal.

Harvest declarations 456 begin with a prologue comment of the form “--!! Harvest Outputs;” and end with a comment of the form “--!! End Harvest Outputs;”. Each harvest event output is associated with a message that may be output by the simulation runtime environment when a harvest event has occurred during a simulation run. Each harvest event signal is declared in the form “--!! n: “harvest message”;” where n is an integer denoting which harvest event the message is to be associated with and “harvest message” is the message to be associated with the particular harvest event. One, and only one, harvest message declaration comment must be provided for each harvest event monitored by the instrumentation entity.

Harvest messages, fail messages, and counter variable names for a simulation model are included in a simulation executable model. In this manner, each simulation model includes the information for each event monitored.

It is advantageous to ensure the uniqueness of these messages and variable names in the event that multiple instances of the same instrumentation entity occur within a simulation model. To this end, the instance identifier of a target entity is pre-pended to each harvest and failure message and each variable name of all instrumentation entities. For example, the first failure message of instance 410 of the FXUCHK entity monitoring instance 321 a (the FXU:FXU0 instantiation) is “FXU0: Fail message for failure event 0”. The instance identifier serves to indicate which particular instance of the FXUCHK entity detected a given failure. In a similar manner, the harvest messages and the variable names for each instrumentation entity instantiation are altered.

Finally, epilogue comment 457 consists of a single comment of the form “--!! End;”, indicating the end of descriptor comments 451. The remainder of instrumentation entity HDL file 440 that follows the I/O declarations described above, is an entity body section 458. In entity body section 458, conventional HDL syntax is utilized to define internal instrumentation logic necessary to detect the various events on the input port signals and convey these events to the output port signals.

In addition to descriptor comments 451, that are located in the HDL source code file for an instrumentation entity, an additional comment line is required in the target entity HDL file. A comment of the form “--!! Instrumentation: name.vhdl”, where name.vhdl is the name of the instrumentation entity HDL file, is added to the target entity HDL source code file. This comment provides a linkage between the target entity and the instrumentation entity that monitors the target entity. It is possible to have more than one such comment in a target entity when more than one instrumentation entity is associated with the target entity. These HDL file comments will hereinafter be referred to as “instrumentation entity instantiations”.

With reference now to FIG. 4D wherein is depicted a model build process in accordance with the teachings of the present invention. In this model build process, instrumentation load tool 464 is utilized to alter the in-memory proto and instance data structures of a simulation model thereby adding instrumentation entities to the simulation model. Instrumentation load tool 464, utilizes descriptor comments 451, in instrumentation HDL files 461, to create instance data structures for the instrumentation entities within a simulation model.

The revised model build process of FIG. 4D begins with design entity HDL files 340 and, potentially, one or more design entity proto files 345, available from a previous run of HDL compiler 462, instrumentation entity HDL files 460, sand, potentially, one or more instrumentation entity proto files 461, available from a previous run of HDL compiler 462. HDL compiler 462, processes design entity HDL files 340, and the instrumentation entity HDL files, 460, following an augmentation of algorithm process_HDL_file( ) that provides for efficient incremental compilation of the design and instrumentation entities comprising a simulation model. HDL compiler 462 loads proto data structures from design entity proto files 345 and instrumentation entity protos files 461 where such proto files are available and consistent. If such proto files are not available or are not consistent, HDL compiler 462 compiles, as needed, design entity HDL files 340 and instrumentation entity HDL files 460 in order to produce design entity proto files 344 and instrumentation entity proto files 468. (design entity proto files 344 and instrumentation entity proto files 468 are available to serve as design entity proto files 345 and instrumentation entity proto files 460 respectively for a subsequent run of HDL compiler 462).

In addition, HDL compiler 462 creates, as necessary, in-memory design proto data structures 463 and design instance data structures 465 for the design entities of a simulation model. HDL compiler 462 also creates in-memory instrumentation proto data structures 466 for the instrumentation entities of a simulation model.

In order to minimize the amount of alterations necessary to allow HDL compiler 462 to perform as depicted in FIG. 4D, HDL compiler 462 neither reads nor processes descriptor comments 451. However, HDL compiler 462 does recognize instrumentation entity instantiation comments within target entity HDL files. As such, HDL compiler 462 cannot create instance data structures for instrumentation entities, depicted in FIG. 4D as instrumentation entity data structures 467. The creation of instance data structures requires interconnection information contained within descriptor comments 451 not processed by HDL compiler 462. HDL compiler 462 does, however, create instrumentation proto data structures 466.

The in-memory design proto data structures 463, design instance data structures 465, and instrumentation entity proto data structures 466 are processed by instrumentation load tool 464. Instrumentation load tool 464 examines design entity proto data structures 463 and design entity instance data structures 465 to determine those design entities that are target entities. In a preferred embodiment of the present invention, this examination is accomplished by utilizing a particular comment format as previously described.

Any target entity that is loaded from design entity proto files 345 contains an instantiation for any instrumentation entities associated with the target entity. Therefore, instrumentation load tool 464 merely creates an instance data structure 467 for any such instrumentation entities and passes, unaltered, the design proto data structure 463 to instrumented design proto data structure 463 a and design instance data structure 465 to instrumented design instance data structure 465 a.

If however, a target entity is loaded from design entity HDL files 340, rather than from design entity proto files 345, instrumentation load tool 464 must alter the design proto data structure 463 and design instance data structure 465 to instantiate the instrumentation entities for the current design entity thereby producing instrumented design proto data structure 463 a and instrumented design instance data structure 465 a. In addition, instrumentation load tool 464 creates an instrumentation instance data structure 467 for each instrumentation entity associated with the current design entity.

Those design entity proto data structures 463, altered by instrumentation load tool 464 are saved to disk 33 of computer system 10 as design entity proto files 344. Design entity proto files 344, which may include references to instrumentation entities, are directly loaded by a subsequent compilation of a simulation model thus saving processing by instrumentation load tool 464 on subsequent recompilations unless an alteration is made to a design entity or an associated instrumentation entity.

In order for HDL compiler 462 to determine if alterations were made to either a target design entity or the target design entity's associated instrumentation entities, the BOM of a target design entity is expanded to include the HDL files constituting the instrumentation entities. In this manner, HDL compiler 462 can determine, by inspection of the BOM for a given design entity, whether to recompile the design entity and the design entity's associated instrumentation entities or load these structures from proto files 345 and 461.

Finally, instrumentation load tool 464 creates a unique proto and instance data structure for instrumentation logic block 420 and connects the fail, harvest, and count event signals from each instrumentation entity instantiation to instrumentation logic block 420. Model build tool 446 processes in-memory proto and instance data structures 463 a, 465 a, 467, 466 to produce instrumented simulation executable model 480

In HDL compiler 462, algorithm process_HDL_file( ) is augmented to allow for the incremental compilation of design and instrumentation entities. A pseudocode implementation of a main control loop of HDL compiler 462 is shown below:

process_HDL_file2(file, design_flag) 5 { 10 if (NOT proto_loaded(file)) { 15 if (exists_proto_file(file) AND check_bom(file)) { 20 load_proto(file); 25 } else { 30 parse_HDL_file(file) 35 for (all instances in file) { 40 process_HDL_file2(instance, design_flag); 45 } 50 if (design_flag = TRUE) { 55 for (all instrumentation instances in file) { 60 process_HDL_file2(instance, FALSE); 65 } 70 } 75 create_proto(file); 80 write_proto_file(file); 90 } 95 } 100 if (design_flag = TRUE) { 105 create_instance(file); 110 } 115 } 120

Algorithm process_HDL_file2( ) is an augmentation to process_HDL_file( ) of HDL compiler 342 in order to support the creation of instrumented simulation models. The algorithm is invoked with the name of the top level design entity passed through parameter file and a flag indicating whether the entity being processed is a design entity or an instrumentation entity passed through parameter design_flag (design flag=TRUE for design entities and FALSE for instrumentation entities). Algorithm process_HDL_file2( ) (line 5) first checks, by means of routine proto_loaded( ) (line 15), if the proto for the current entity is already present in memory 44. If so, processing passes to line 105. Otherwise, control is passed to line 20 and 25 where disk 33 of computer system 10 is examined to determine if proto files for the entity and its descendants (including instrumentation entities, if any) exist and are consistent. If so, the appropriate proto files are loaded from disk 10 by routine load_proto( ) (line 25) creating proto data structures, as necessary, in memory 44 for the current entity and the current entity's descendants including instrumentation entities.

If the proto file is unavailable or inconsistent, control passes to line 35 where the current entity HDL file is parsed. For any entities instantiated within the current entity, lines 40 to 55 recursively call process_HDL_file2( ) (line 5) in order to process these descendants of the current entity. Control then passes to line 55 where the design_flag parameter is examined to determine if the current entity being processed is a design entity or an instrumentation entity. If the current entity is an instrumentation entity, control passes to line 80. Otherwise, the current entity is a design entity and lines 60 to 70 recursively call process_HDL_file2( ) (line 5) to process any instrumentation entities instantiated by means of instrumentation instantiation comments. It should be noted that algorithm process_HDL_file2( ) (line 5) does not allow for instrumentation entities to monitor instrumentation entities. Any instrumentation entity instantiation comments within an instrumentation entity are ignored. Control then passes to line 80 where proto data structures are created in memory 44 as needed for the current entity and any instrumentation entities. Control then passes to line 90 where the newly created proto data structures are written, as needed to disk 33 of computer system 10.

Control finally passes to line 105 and 110 where, if the current entity is a design entity, instance data structures are created as needed for the current entity and the current entity's descendants. If the current entity is an instrumentation entity, routine create_instance( ) (line 110) is not called. Instrumentation load tool 464 is utilized to create the in-memory instance data structures for instrumentation entities.

It will be apparent to those skilled in the art that HDL compiler 462 provides for an efficient incremental compilation of design and instrumentation entities. It should also be noted that the above description is but one of many possible means for accomplishing an incremental compilation of instrumentation entities. In particular, although many other options also exist, much, if not all, of the functionality of instrumentation load tool 464 can be merged into HDL compiler 462.

With reference now to FIG. 4E wherein is shown a depiction of memory 44 at the completion of compilation of simulation model 329 with instrumentation entities FXUCHK and FPUCHK. Memory 44 contains proto data structures 481, one for each of the design and instrumentation entities referred to in simulation model 329. In addition, design and instrumentation instances in simulation model 329 are represented by instance data structures 482. The instance data structures are connected by means of pointers indicating the hierarchical nature of the instantiations of the design and instrumentation entities within simulation model 329.

With reference now to FIG. 5A, wherein is depicted failure flags 424 of instrumentation logic block 420 in greater detail. Failure flags 424 consist of registers 500 a-500 n utilized to accept and store an indication of the occurrence of a failure event. In what follows, the operation of a single failure flag for a particular failure event 502 will be discussed. The operation of all failure flags is similar.

Register 500 a holds a value that represents whether a failure event 502 has occurred or not. Register 500 a is initially set to a value of ‘0’ by the simulation run-time environment at the beginning of a simulation run. When failure event 502, if enabled at register 507 a, occurs, register 500 a is set to a value of a logical ‘1’, thereby indicating the occurrence of a failure event. Register 500 a is driven by logical OR gate 501. Logical OR gate 501 performs a logical OR of the output of register 500 a and a qualified failure signal 503 to create the next cycle value for register 500 a. In this manner, once register 500 a is set to a logical ‘1’ by the occurrence of an enabled failure event, register 500 a maintains the value of a logical ‘1’ until reset by the simulation runtime environment. Likewise, register 500 a maintains a value of ‘0’ from the beginning of the simulation run until the occurrence of the failure event, if enabled.

Qualified failure signal 503 is driven by logical AND gate 505. Logical AND gate 505 produces, on qualified failure signal 503, the logical AND of failure signal 506 and the logical NOT of register 507 a. Register 507 a serves as an enabling control for qualified failure signal 503. If register 507 a contains a value of ‘0’, logical AND gate 505 will pass failure event signal 506 unaltered to qualified failure signal 503. In this manner, the monitoring of the failure event is enabled. Registers 507 a-507 n are set, by default, to a value of ‘0’. However, if register 507 a contains a value of a logical ‘1’, qualified failure signal 503 will remain at a value of ‘0’ irrespective of the value of failure event signal 506, thereby disabling the monitoring of failure event 502. In this manner, register 508, consisting of registers 507 a-507 n, can mask the occurrence of any subset of failure events in the overall simulation model from registers 500 a-500 n.

To efficiently implement the ability to selectively disable the monitoring of failure events, the simulation run-time environment includes a function that allows a user to disable monitoring of a specific failure event for a given instrumentation entity. This function will automatically set the appropriate registers among registers 507 a-507 n within register 508 to disable the monitoring of a particular failure event for every instance of the instrumentation entity within the overall simulation model. Instrumentation load tool 464 and model build tool 446 encode sufficient information within instrumented simulation executable model 480 to determine which failure bits within register 508 correspond to which instrumentation entities.

The ability to selectively disable monitoring of failure events is of particular use in large batch-simulation environments. Typically, in such an environment, a large number of general purpose computers, running software or hardware simulators, are dedicated to automatically running a large number of simulation runs. If a simulation model with a faulty instrumentation entity that incorrectly indicates failure events is run in such an environment, a large number of erroneous failures will be generated causing lost time. By selectively disabling failure events within instrumentation entities, the present invention allows simulation to continue while only disabling erroneous failure signals rather than having to disable all failure monitoring. This option is particularly useful when the process of correcting a faulty instrumentation entity and creating a new simulation model is substantially time consuming. The present invention also provides similar enabling and disabling structures for the harvest and count events within a model.

Logical OR gate 512 is utilized to produce a signal, 511, that indicates whether any failure event within the model has occurred. This signal is utilized to allow hardware simulators to efficiently simulate simulation models that have been instrumented according to the teachings of the present invention.

With reference now to FIG. 5B there is illustrated in greater detail, features of the present invention utilized to support efficient execution of an instrumented simulation model on a hardware simulator. It should be noted that for most hardware simulators, the operation of polling a facility within a simulation model during a simulation run is often a time consuming-operation. In fact, if facilities must be polled every cycle, it is often the case that as much, if not considerably more, time is spent polling a simulation model for results rather than running the actual simulation. As such, it is advantageous when using a hardware simulator to avoid polling facilities within the model during a simulation run. In addition, many hardware simulators provide a facility that instructs the hardware simulator to run a simulation without interruption until a specific signal within the simulation model attains a specific value. This facility usually results in the highest performance for a simulation run on a hardware simulator.

In order to execute simulation model 520 on a hardware simulator, a termination signal 513, is typically utilized as a means to avoid having to poll the model after each cycle. Typically, a hardware simulator will cycle simulation model 520 until signal 513 is asserted to a logical ‘1’. The assertion of termination signal 513 to a logical ‘1’ indicates that a simulation run has finished. Without termination signal 513, it would be necessary to directly poll facilities within simulation model 520 to determine when a simulation run is completed.

To efficiently locate and diagnose problems in simulation model 520, it is advantageous to allow a simulation run to be stopped immediately whenever a failure event occurs during simulation of simulation model 520 (harvest events and count events are typically only polled at the end of a simulation run). This allows a user to easily locate the failure event within the simulation run, thereby facilitating debugging of the failure. In order to allow simulation models that have been instrumented according to the teachings of the present invention to efficiently execute on a hardware simulator, a comment of the form “--!! Model Done: signalname” is placed within the HDL source code file for the top level entity of the simulation model where signalname is the name of termination signal 513 within the simulation model. This comment is only utilized if present in the HDL file for the top-level entity. If such a comment is present in the HDL source code file for the top level entity, a logical OR gate 515 will automatically be included within the simulation model. Logical OR gate 515 produces the logical OR of signals 511 and 513 on signal 516. Signal 516 is therefore asserted to a logical ‘1’ whenever the simulation run has completed (signal 513 high) or a failure event has occurred (signal 511 high). Consequently, by executing simulation model 520 in a hardware simulator until signal 516 is asserted to a value of a logical ‘1’, the instrumentation for simulation model 520 can be combined and utilized along with existing simulation termination techniques in a seamless manner. In the alternative, if the comment indicating the name of termination signal 513 is not present, logical OR gate 515 is not included in the model and signal 511 is directly connected to signal 516. The name of signal 516 is fixed to a particular name by convention.

In many simulators, the passage of time within the simulated model is modeled on a cycle-to-cycle basis. That is to say, time is considered to pass in units known as cycles. A cycle is delineated by the occurrence of a clock signal within a simulation model that regulates the updating of storage elements within the design. These simulators are commonly known as “cycle simulators”. A cycle simulator models a digital design by repeatedly propagating the values contained within storage elements through interconnecting logic that lies between storage elements without specific regard for the physical timing of this propagation, to produce next cycle values within the storage elements. In such simulators, a primitive storage element, hereinafter referred to as a “simulator latch”, is utilized to model the storage elements within a digital design. One simulator cycle therefore consists of propagating the current values of the simulator latches through the interconnecting logic between storage elements and updating the simulator latches with the next cycle value.

In many circumstances, however, it is not possible to utilize a single simulator latch to directly model the storage elements within a design. Many common storage elements utilized within digital designs often require more than one simulator latch. For example, so called master-slave flip-flops are generally modeled utilizing two simulator latches to accurately simulate the behavior of such storage elements. In order to efficiently model storage elements, a designer will typically refer to a library that contains storage element simulation models for use in a design. These design storage elements are modeled by one or more simulator latches. Storage elements comprised of one or more simulator latches that are implemented within a design will be referred to hereinbelow as “design latches”.

As a consequence of utilizing multiple simulator latches to model a design latch, the process of propagating the input of a design latch to its output, which constitutes a design cycle, often requires more than one simulator cycle. A single design cycle is thus defined as comprising the number of simulator cycles required to propagate a set of values from one set of storage elements to the next.

In other circumstances, a simulation model may consist of distinct portions that are clocked at differing frequencies. For example, a microprocessor core connected to a bus interface unit, may operate at a higher frequency and than the bus interface unit. Under these circumstances, the higher frequency portion of the design will require one or more simulator cycles, say N cycles, to simulate a single design cycle. The lower frequency portion of the design will require a multiple of N simulator cycles in order to simulate a design cycle for the lower frequency portion. This multiple is equal to the ratio of the frequency of the higher speed design portion to the frequency of the lower speed design portion. It is often the case that certain portions of the logic can be run at a number of differing frequencies that are selectable at the beginning of a simulation run. Such logic, with a run-time selectable frequency of operation, presents unique challenges for monitoring count events.

With reference now to FIG. 6A, there is depicted a gate level representation of exemplary logic for one counter of counters 421 within instrumentation logic block 420 depicted in FIG. 4B. Each counter of 421 is represented by a multi-bit simulator latch 600. Simulator latch 600 is initialized by the simulation runtime environment to a value of zero at the beginning of a simulation run. Simulator latch 600 is updated every simulator cycle and is driven by multiplexor 601. Multiplexor 601, controlled by selector signal 602, selects between signal 603, the current value of simulator latch 600, and signal 605, the current value of simulator latch 600 incremented by 1 by incrementor 604, to serve as the next cycle value for simulator latch 600. By selecting signal 605, multiplexor 601 causes the counter value within simulator latch 600 to be incremented when a count event occurs. It should be noted, however, that simulator latch 600 is updated every simulator cycle irrespective of the number of simulator cycles that correspond to a design cycle for the logic being monitored by a counting instrument. Logical AND gate 606 and simulator latch 607 serve to disable the monitoring of count event signal 609 in a manner similar to that described above for the disabling of failure events. Signal 608 is count event signal 609 further qualified by signal 610 by means of logical AND gate 611.

Signal 610 insures that simulator latch 600 will be incremented, if count event signal 609 is active, only once per design cycle for the logic being monitored by a counting instrument irrespective of the number of simulation cycles utilized to model the design cycle. This clocking normalization is necessary to ensure that the event counts recorded in counters 421 correspond directly to the number of design cycles the event occurred in and not the number of simulator cycles the event occurred in. For example if an event occurs in two design cycles where design cycle require four simulators cycles, it is preferable to have the event counter reflect a value of two rather than a value of eight as would occur if the counter were allowed to update in every simulator cycle.

Furthermore, if the count event being monitored is within a portion of the logic with a run-time selectable frequency of operation, it is useful to have the count registers reflect the number of occurrences of the event in terms of design cycles. For example, consider a circumstance where a count event occurs twice during two different simulation runs. In the first run, assume that four simulator cycles are needed to represent each design cycle. Further assume in the second run that twelve simulator cycles are necessary to represent each design cycle. Without a clocking normalization mechanism, the first run would indicate that the event occurred eight times (two occurrences times four simulator cycles per occurrence) and the second run would indicate that the event occurred twenty-four times (two occurrences times twelve simulator cycles per occurrence) when in fact the event actually only occurred twice in both simulation runs. Therefore, it would be advantageous to limit the updating of counters 421 such that each counter is only updated once per design cycle irrespective of the number of simulator cycles, possibly variable at run-time, needed to represent a design cycle.

In simulation models in which multiple simulator cycles are utilized to represent a single design cycle, explicit clocking signals are utilized within the model to control the updating of the various design storage elements. These clocking signals specify in which simulator cycles the simulator latches representing design storage elements are allowed to update. A clocking signal is asserted high for some contiguous number of simulator cycles either at the beginning or end of the design cycle and asserted low for the remaining simulator cycles within the design cycle. If the clocking signal is asserted high during the beginning of the design cycle, the clock is referred to as a “high-active” clock and, likewise, if the clocking signal is asserted low during the beginning of the design cycle, the clock is referred to as a “low-active” clock.

Each count event signal has an associated qualifying signal as specified by counter declaration comments 455 as described above. Typically, these qualifying signals are connected to the clocking signals within the design responsible for updating the storage elements within the portion of logic monitored by the count event. The qualifying signal for the count event for simulator latch 600, qualifying signal 612, is depicted as a high-active qualifier signal. Qualifying signal 612 is processed by simulator latch 613 and logical AND gate 614, to produce signal 610 which is active high for one and only one simulator cycle within the design cycle delineated by qualifying signal 612.

Turning now to FIG. 6B there is illustrated a simplified timing diagram that demonstrates operation of simulator latch 613 and logical AND gate 614 assuming clocking qualifying signal 612 is a high active clocking signal of fifty percent duty cycle for a design cycle that occurs over a 10-simulation cycle period. Signal 615, the output of simulator latch 613, is qualifying signal 612 delayed by one simulator cycle. Signal 615 is inverted and logically ANDed with qualifying signal 612 to produce signal 610, a high-active pulse that is asserted for the first simulator cycle of the design cycle. In a similar fashion, if the qualifying clock signal is low active, qualifying signal 612 would be inverted and signal 615 would be uninverted by logical AND gate 614. This would produce a single simulator cycle active high pulse during the first simulator cycle of the design cycle. Qualifying signal 610, by qualifying count event signal 609 by means of logical AND gate 611, insures that counter 600 is incremented only once per design cycle irrespective of the number of simulator cycles utilized to represent a design cycle.

Returning to FIG. 6A, incrementor 604 represents but one possible mechanism that may be utilized to implement the next logic state for a given counter within the present invention. As depicted in FIG. 6A, incrementor 604 ensures that counters 421 within a model are cycled through a series of values whose binary patterns correspond to the customary representation of non-negative integers. In one embodiment of the present invention, incrementor 604 is comprised of an adder that increments the current value of counter 600 by a unit value each time signal 605 is selected by selector signal 602, This exemplary implementation provides for convenience of decoding the value of counter 600 at the termination of a simulation run, but does so at a cost in overhead that is not acceptable in many simulators.

Returning to FIG. 6, incrementor 604 represents but one possible mechanism that may be utilized to implement the next logic state for a given counter within the present invention.

As depicted in FIG. 6, incrementor 604 ensures that counters 421 within a model are cycled through a series of values whose binary patterns correspond to the customary representation of non-negative integers. In one embodiment of the present invention, incrementor 604 is comprised of an adder that increments the current value of counter 600 by a unit value each time signal 605 is selected by selector signal 602. This exemplary implementation provides for convenience of decoding the value of counter 600 at the termination of a simulation run, but does so at a cost in overhead that is not acceptable in many simulators.

For software simulators, one of two basic approaches may be utilized to model an incrementor, such as incrementor 604. In the first approach, the incrementor is modeled directly by an ADD or INCREMENT instruction in the simulation execution model. When incrementors are modeled directly as a single instruction within the simulation execution model, the use of incrementor 604 provides for efficient counters within a simulation execution model.

However, many software simulators and virtually all hardware simulators model incrementor functions as a set of gates that are replicated essentially without change at each bit position of the counter. Within a software simulator, these gates must be translated into a sequence of instructions. In a hardware simulator, these gates are explicitly replicated for each counter as individual gates. Due to implementation or structural limitations, many software simulators are incapable of modeling an incrementor in any other manner than as a set of gates. Clearly, for these software simulators that must model incrementors as a number of gates and therefore as a sequence of instructions, a performance loss will result over those software simulators that model incrementors as a single increment or add instruction. Likewise, for hardware simulators, the number of gates required for each adder, which must be modeled directly by gates within the hardware simulator, can prove to be a significant burden.

The method and system of the present invention alleviate these difficulties by implementing a linear feedback shift register as the counting means within counting instrumentation. As explained below, appropriate configuration and utilization of such a liner feedback shift register results in an efficient method of incrementing a counter that avoids the overhead associated with incrementor 604.

With reference now to FIG. 7, there is depicted a linear feedback shift register (LFSR) counter 700 consisting of a shift register 704 and “exclusive NOR” (XNOR) gate 706. Various methods of constructing LFSRs are known to those skilled in the art. As illustrated in FIG. 7, LFSR counter 700 includes a modified shift register 704 that may replace register 600 and incrementor 604 of FIG. 6. LFSR counter 700 also includes multiplexor 601 (replicated bit-by-bit within LFSR 704) which provide feedback paths 616. Feedback path 616 provides a means for shift register 704 to maintain its current value during those simulator cycles in which no count pulse trigger (signal 602) is received. For hardware and software design simulators in which, for logistical or other reasons, incrementation of counters must be accomplished utilizing a set of gates for each counter, shift register 704 replaces register 600 within the counter logic depicted in FIG. 6. The need for incrementor 604 is thus eliminated and is replaced by XNOR gate 706. In this manner, register 600 and incrementor 604 are replaced utilizing a more efficient logic structure having substantially reduced overhead. Counters 421 of FIG. 4B, will therefore consist of LFSR-based configurations such as LFSR counter 700 whose values can be decoded at the end of a simulation run to reveal their corresponding integral values.

Shift register 704 can be of any desired length. In a preferred embodiment, shift register 704 is a 22 bit register, although larger or smaller registers may be employed. Shift register 704 consists of latches 718 arranged in a serial fashion such that a given latch's output is utilized as input to the next latch 718 within shift register 704. In addition, a select subset of latches 718 within shift register 704 have their outputs sourced to XNOR gate 706. XNOR gate 706 is utilized to provide an input for the first latch within shift register 704.

The LFSR is a logic structure that, when properly configured, will sequence through all possible bit patterns with the exception of the all-ones pattern (it is possible to construct LFSRs which exclude the all-zeros pattern or LFSRs that cycle through all possible bit patterns) . For example, in a22 bit LFSR, bits 1 and 22 may be selected for inputs to XNOR gate 706 to provide a sequence of bit patterns in shift register 704 which traverses every possible permutation with the exception of the all-ones pattern. Shift register 704 must be loaded with an initial value that is not the all ones pattern. This may be accomplished automatically by initializing all latches to a binary zero value within the simulator, or by utilizing the control program that drives the simulator to explicitly set these latches to binary zeros.

After initialization, the numeric pattern held by bit positions 718 of shift register 704 will cycle through a specific and predictable pattern in a repeating fashion. That is to say, for any given bit pattern present in shift register 704, there is a specific, unique pattern the shift register will subsequently assume upon being shifted and therefore, the sequence of patterns through which the shift register cycles is fixed and repeats in a predictable manner. Due to these properties, LFSR counter 700 can be utilized as a counting means within for the instrumentation detection means previously described. By assigning the value of “zero” to a pre-selected starting value (say the all zeros pattern for shift register 704), the value of “one” to the next bit pattern formed by shifting the LFSR, and so on, the LFSR can serve as a counter. To be useful as a counter, the bit patterns within shift register 704 must be converted back to their corresponding integer values. This is easily accomplished for LFSRs with a small number of bits (less than 25 bits) by means of a lookup table consisting of an array of values, where the index of the array corresponds to the LFSR bit pattern value and the entry in the array is the decoded integer value for the LFSR. For LFSRs with a larger number of bits, software decoding techniques can be utilized to decode the LFSR value by simulating the operation of the LFSR.

As illustrated in FIG. 7, the logic necessary to implement LFSR counter 700 consists of the single XNOR gate 706 with two feedback inputs. While the number of required feedback gates and inputs thereto may vary in proportion to different possible lengths of an LFSR, in general, for typical LFSRs (less than 200 bits), only one XNOR gate with a relatively small number of inputs (less than 5 bits) is required. This is in marked contrast to the several logic gates per bit required for conventional incrementors. Therefore, significant savings in counter overhead can be achieved by substituting LFSR-based counter 700 for the incrementor structure depicted in FIG. 6, especially for simulators that model incrementors utilizing logic gate based representations.

While the invention has been particularly shown as described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention. One of the embodiments of the invention can be implemented as sets of instructions resident in random access memory 28 of one or more computer systems configured generally as described in FIG. 1 and FIG. 2. Until required by computer system 10, the set of instructions may be stored in another computer readable storage device, such as disk drive 33 or in a removable storage device such as an optical disk for eventual use in a CD-ROM drive or a floppy disk for eventual use in a floppy disk drive. The set of instructions may be referred to as a computer program product. Further, the set of instructions can be stored in the memory of another computer and transmitted over a local area network or a wide area network, such as the Internet, when desired by the user. It is therefore contemplated that such modifications can be made without departing from the spirit or scope of the present invention as defined in the appended claims. 

What is claimed is:
 1. A method within a computer-aided design and verification system for counting events within a simulation model of a digital circuit design during simulation of said digital circuit design, said method comprising the steps of: describing a design entity utilizing a hardware description language, said design entity forming part of a digital circuit design; and instantiating a counting instrument within said design entity utilizing said hardware description language, said counting instrument described by said hardware description language to detect occurrences of a count event within said design entity during simulation of said digital circuit design, wherein said instantiating a counting instrument includes generating a linear feedback shift register associated with said counting instrument, said linear feedback shift register utilized for recording occurrences of said count event within said design entity.
 2. The method of claim 1, further comprising the steps of: instantiating said design entity in at least one instance in a simulation model of said digital circuit design; and associating said counting instrument with said design entity utilizing a non-conventional comment, such that said counting instrument may be utilized to monitor said at least one instance of said design entity without said counting instrument becoming incorporated into said digital circuit design.
 3. The method of claim 2, wherein said non-conventional comment is contained within a hardware description language source code file, said hardware description language source code file utilized to describe said counting instrument.
 4. The method of claim 1, wherein said linear feedback shift register is comprised of a plurality of latches connected in series, said method further comprising the step of initializing said series of latches to a pre-determined initial value.
 5. The method of claim 4, wherein said linear feedback shift register further includes a shift register comprised of said plurality of serially coupled latches and an XNOR gate, said method further comprising the step of providing feedback from the outputs of at least two of said latches into said shift register through said XNOR gate.
 6. The method of claim 4, wherein said simulation is performed in accordance with a simulator cycle and said design entity operates in conformity with a design cycle, said design cycle consisting of a multiple of said simulator cycle, said method further comprising the step of encoding said design cycle within said counting instrument, such that said linear feedback shift register is automatically adjusted to count in conformity with said design cycle.
 7. The method of claim 6, wherein said multiple is equal to the number of simulator cycles required to propagate a set of values between storage elements within said design entity.
 8. The method of claim 6, further comprising the step of detecting count events within said counting instrument.
 9. The method of claim 8, further comprising the step of delivering said detected count events to said linear feedback shift register.
 10. The method of claim 9, further comprising the step of recording said delivered count events within said linear feedback shift register.
 11. A program product stored in signal bearing media within a computer-aided design and verification system for counting events within a simulation model of a digital circuit design during simulation of said digital circuit design, said program product comprising: instruction means for describing a design entity utilizing a hardware description language, said design entity forming part of a digital circuit design; instruction means for instantiating a counting instrument within said design entity utilizing said hardware description language, said counting instrument described by said hardware description language to detect occurrences of a count event within said design entity during simulation of said digital circuit design, wherein said instruction means for instantiating a counting instrument within said design entity includes instruction means for generating a linear feedback shift register associated with said counting instrument, said linear feedback shift register utilized for recording occurrences of said count event within said design entity.
 12. The program product of claim 11, further comprising: instruction means for instantiating said design entity in at least one instance in a simulation model of said digital circuit design; and instruction means for associating said counting instrument with said design entity utilizing a non-conventional comment, such that said counting instrument may be utilized to monitor said at least one instance of said design entity without said counting instrument becoming incorporated into said digital circuit design.
 13. The program product of claim 12, wherein said non-conventional comment is contained within a hardware description language source code file, said hardware description language source code file utilized to describe said counting instrument.
 14. The program product of claim 11, wherein said linear feedback shift register is comprised of a plurality of latches connected in series, said program product further comprising instruction means for initializing said series of latches to a pre-determined initial value.
 15. The program product of claim 14, wherein said linear feedback shift register further includes a shift register comprised of said plurality of serially coupled latches and an XNOR gate, said program product further comprising instruction means for providing feedback from the outputs of at least two of said latches into said shift register through said XNOR gate.
 16. The program product of claim 14, wherein said simulation is performed in accordance with a simulator cycle and said design entity operates in conformity with a design cycle, said design cycle consisting of a multiple of said simulator cycle, said program product further comprising instruction means for encoding said design cycle within said counting instrument, such that said linear feedback shift register is automatically adjusted to count in conformity with said design cycle.
 17. The program product of claim 16, wherein said multiple is equal to the number of simulator cycles required to propagate a set of values between storage elements within said design entity.
 18. The program product of claim 16, further comprising instruction means for detecting count events within said counting instrument.
 19. The program product of claim 18, further comprising instruction means for delivering said detected count events to said linear feedback shift register.
 20. The program product of claim 19, further comprising instruction means for recording said delivered count events within said linear feedback shift register. 